Patent application WO 2011/009 560 describes stable aqueous dispersions containing core/shell two-phase structured particles with a hard core P1 with a Tg1 from 60 to 120° C. and a soft shell P2 with a Tg2 from −20 to 40° C. and an MFT from 0 to 50° C. with P1 representing from 15% to 60% out of P1+P2, for applications in aqueous coatings, which are in particular self-crosslinkable. Despite the very good capacity for film formation and the control of this process with good reproducibility of the film formation and the performance qualities, these dispersions have insufficient performance qualities for certain applications, in particular in terms of hardness, blocking resistance and imprint resistance with better cohesion and mechanical strength of the film obtained. More particularly, the hardness, the blocking resistance and the imprint resistance of coatings of this type are insufficient (too soft despite the reinforcement of the hard core) for certain applications requiring such performance qualities, in particular in industrial applications in which coated heavy objects (doors, windows, furniture, plate metals) are rapidly stacked after applying the coating or decorative applications which require good heat resistance (kitchens, bathrooms) or good imprint resistance (high-resistance wall paints, resistant to scratching and to abrasion).
In point of fact, WO 2011/009 560 describes the structuring of a hard-core soft-shell latex particle according to a specific process. Such a structure leads to a polymer film (coating) which behaves in the same way as would a soft rubber phase reinforced with a dispersion ideally made from hard organic particles which act as reinforcing nanofillers and which are synthesized in situ in a first step. The film thus obtained is flexible (continuous soft phase) and the mechanical reinforcement by the regular inclusions of hard organic nanofillers (core) is reflected by a tensile stress that increases with the elongation, representative of a gradual reinforcement. The soft phase is the continuous phase and, as a result, the quality of the film formation is very good with a homogeneous film. The mechanical properties of the film, obtained by virtue of the perfect film formation, are stable over time with good durability. The process described makes it possible to obtain a predictable, reproducible MFT that is stable over time. However, depending on the industrial application conditions, the required levels of hardness and of blocking resistance and imprint resistance are difficult to achieve with a soft-shell particle which ultimately constitutes the continuous phase of the film.
Other solutions are also described with polymer dispersions containing structured polymer particles, with a soft core and a hard shell, making it possible to obtain high hardness and blocking resistance. However, owing to its particle structure, this type of polymer dispersion leads to a film that is relatively sparingly cohesive since the continuous phase which is hard and crosslinked rapidly yields when it is stressed. Finally, the presence of a hard shell around the particle, when a high level of gloss is desired, requires the use of coalescers despite the low MFT of the emulsion.